A duplication has been one of the most established tactical schemes in designing important parts of an electronic switching system. The electronic switching system becomes more reliable by employing the duplication.
In a duplicated system, a couple, or more than two in certain events, of subsystems form duplicated mates and the duplicated mates operate alternatively by swinging between an active mode and a stand-by mode. Typically, one of the duplicated mates, the one in the active mode, operates normally while the other in the stand-by mode is waiting ready for a switch-over to an active mode. Conventionally, logic devices have been used for determining the active mode and the stand-by mode of the duplicated mates.
Conventional apparatus for determining the active mode and the stand-by mode of the duplicated system is given in FIGS. 1 and 2.
FIG. 1 shows a half of the apparatus, employed in each subsystem in the duplicated system, for determining the active and the stand-by mode of the duplicated system.
A S.sub.-- FUN.sub.-- FAIL(self function fail) is a signal indicating whether a subsystem is in a normal condition or is in a functionally failed condition. A S.sub.-- ACTIVE(self active) is a signal telling whether a subsystem in the duplicated system is in the active mode or in the stand-by mode; an X.sub.-- ACTIVE (external active) is a signal signifying whether a counterpart of a subsystem in the duplicated system is in the active mode or in the stand-by mode.
For the purpose of illustration, it is assumed that: a subsystem is in the active mode when the S.sub.-- ACTIVE is "low", in the stand-by mode when the S.sub.-- ACTIVE is "high"; a counterpart is in the active mode when the X.sub.-- ACTIVE is "low", in the stand-by mode when the X.sub.-- ACTIVE is "high"; a subsystem is in the normal condition when the S.sub.-- FUN.sub.-- FAIL is "high", in the functionally failed condition when the S.sub.-- FUN.sub.-- FAIL is "low".
When a subsystem is turned on, it may be designed that the X.sub.-- ACTIVE is set to "high". It is assumed that the S.sub.-- FUN.sub.-- FAIL is "high". An output of an AND gate 11 becomes "high". Then, regardless of an output of an AND gate 12, the S.sub.-- ACTIVE, an output of a NOR gate 13, becomes "low". A feedback through an inverter 14 does not affect states of outputs of the NOR gate 13.
If the S.sub.-- FUN.sub.-- FAIL is "low", which suggests that the apparatus is in the functionally failed condition, outputs of the AND gates 11, 12 are "low", which, in turn, produces a result that the S.sub.-- ACTIVE is "high". The subsystem enters into the stand-by mode.
As can be shown from the conventional apparatus in FIG. 2, each subsystem, the A side or the B side, receives a S.sub.-- ACTIVE signal of its counterpart and regards the S.sub.-- ACTIVE as the X.sub.-- ACTIVE of its own. Upon a power-on, both the S.sub.-- ACTIVE and the S.sub.-- FUN.sub.-- FAIL determine the active and the stand-by modes, and the determined mode remains intact as long as the S.sub.-- FUN.sub.-- FAIL remains "high". All the operation of the conventional apparatus is asynchronous.
The conventional duplicated system may come across a conflict that the two of the duplicated mates are in a same mode since it operates asynchronously. For example, when the X.sub.-- ACTIVE is "low", which suggests that the counterpart subsystem is in the active mode, and the S.sub.-- FUN.sub.-- FAIL is "high", the S.sub.-- ACTIVE is in a logically ambiguous condition, in other words, the S.sub.-- ACTIVE can be either "high" or "low". In the event that the S.sub.-- AVTIVE is "low", a conflict of bidding the same active mode occurs. The conflict may bring about a halt in the operation of the duplicated system. The conventional apparatus, however, provides no means for resolving this conflict.